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In this work, step-by-step high-pressure high-temperature annealing of plastically deformed-type IaAB diamonds from the Istok placer has been performed and the behavior of characteristic centers has been investigated by infrared, electron paramagnetic resonance, and photoluminescence spectroscopies. A new paramagnetic center with one magnetically inequivalent position and hyperfine structure from one hydrogen atom has been found. This paramagnetic center can be assigned to the dangling carbon bond in the dislocation core, and the hydrogen hyperfine structure is due to the polarization of the electron shell of hydrogen forming the C–H bond. For this center and W7 center, the annealing temperatures have been determined. Spin relaxation was studied by pulse electron paramagnetic resonance technique at low temperatures. The spin–spin relaxation times for the W7 and 490.7 centers exclude exchange interactions between the paramagnetic centers. At the same time, the close spin–lattice relaxation times for these centers can be determined by the distorted structure near the dislocation core.

The eclogites of the North Muya complex (Eastern Siberia) are located within the Early Neoproterozoic metasedimentary and felsic rocks of the Baikal-Muya Fold Belt (BMFB). The eclogites show subduction-related affinity, with large-ion lithophile (LILE) and light rare-earth element (LREE) enrichment and high field-strength element (HFSE) depletion signatures, similar to the exposed plutonic and volcanic rocks of the Early Neoproterozoic (Early Baikalian) subduction setting in the BMFB. Coupled Nd (eNd(T) of +6 to −1.4) and Sr (87Sr/86Sr ratio of 0.705–0.708), along with key trace-element indicators, imply progressive crustal recycling (up to 5–10%) from the Early Precambrian continental rocks to a depleted mantle source or equivalent crustal contribution via intracrustal contamination. Mineral δ18O data (+3.9 − +11.5) indicate that the contaminant or recycled crustal substrate might be represented by rocks altered at both low and high-temperature, or result from variable fluid-rock interaction in the subduction channel. Pseudosection modelling of eclogites, coupled with zircon U Pb geochronology (~630 Ma) suggest that the Ediacarian high-pressure metamorphic event for different rocks shared a maximum depth corresponding to 2.5–2.7 GPa with variable temperature range (560–760 °C), reflecting their potential relation to distinct slices of the subducted crust. The estimated metamorphic conditions for both the burial and exhumation of rocks indicate a continental subduction setting, but with a relatively cold geotherm (~20–25 °C/kbar). These conditions resulted from the continental subduction of the Baikal-Muya composite structure beneath the relatively thin and immature overlying arc lithosphere of southern Siberia. Some carbonate-bearing eclogites and garnet-pyroxene rocks, metamorphosed under T below 700 °C and a minimum P up to 1.4 GPa, exhibit LREE-enriched patterns and low eNd(T) values of −7 to −16. These rocks have Paleoproterozoic to Archean model ages and may support the existence of a Paleoproterozoic or older lithosphere in the Baikal-Muya Fold Belt, but their subduction history and origin remain uncertain due to geochemical and isotopic signatures probably overprinted by carbonate metasomatism.

The specific features of the mineralogy of SiO 2 inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ 13 С. SiO 2 inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO 3 , jeffbenite and ferropericlase. All SiO 2 inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO 2 inclusions in sublithospheric diamonds (up to 12.9 δ 18 O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ 18 O of SiO 2 inclusions and δ 13 C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.

The paper describes mineralogical characteristics of SiO2 inclusions in sublithospheric diamonds, which typically have complicated growth histories showing alternating episodes of growth, dissolution, and postgrowth deformation and crushing processes. Nitrogen contents in all of the crystals do not exceed 71 ppm, and nitrogen is detected exclusively as B-defects. The carbon isotope composition of the diamonds varies from δ13С = –26.5 to –6.7‰. The SiO2 inclusions occur in association with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite, and ferropericlase. All SiO2 inclusions are coesite, which is often associated with micro-blocks of kyanite in the same inclusions. It was suggested that these phases have been produced by the retrograde dissolution of primary Al-stishovite, which is also evidenced by the significant internal stresses in the inclusions and by deformations around them. The oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (δ18O up to 12.9‰) indicates a crustal origin of the protoliths. The negative correlation between the δ18O of the SiO2 inclusions and the δ13C of their host diamonds reflects interaction processes between slab-derived melts and reduced mantle rocks at depths greater than 270 km.

According to present views, the crustal terranes of the Anabar province of the Siberian craton were initially independent blocks, separated from the convecting mantle at 3.1 (Daldyn terrane), 2.9 (Magan terrane) and 2.5 Ga (Markha terrane) (Rosen, 2003, 2004; Rosen et al., 1994, 2005, 2009). Previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the Markha terrane concluded that the evolution of the crust of the Markha terrane is very similar to that of the Daldyn terrane. To test this conclusion we present results of U-Pb and Hf-isotope studies on zircons in crustal xenoliths from the Zapolyarnaya kimberlite pipe (Upper Muna kimberlite field), located within the Daldyn terrane, and the Botuobinskaya pipe (Nakyn kimberlite field) in the center of the Markha terrane. The data on xenoliths from the Botuobinskaya kimberlite pipe record tectonothermal events at 2.94, 2.8, 2.7 and 2 Ga. The event at 2 Ga caused Pb loss in zircons from a mafic granulite. U-Pb dating of zircons from the Zapolyarnaya pipe gives an age of 2.7 Ga. All zircons from the studied crustal xenoliths have Archean Hf model ages ranging from 3.65 to 3.11 Ga. This relatively narrow range suggests that reworking of the ancient crust beneath the Nakyn and Upper Muna kimberlite fields was minor, compared with the Daldyn and Alakit-Markha fields (Shatsky et al., 2016). This study, when combined with dating of detrital zircons, implies that tectonic-thermal events at 2.9–2.85, 2.75–2.7 and 2.0–1.95 Ga occurred everywhere on the Anabar tectonic province, and could reflect the upwelling of superplumes at 2.9, 2.7 and 2 Ga. The presence of the same tectonic-thermal events in the Daldyn and Markha terranes (Rosen et al., 2006a,b) supports the conclusion that the identification of the Markha terrane as a separate unit is not valid.

In recent years, despite significant progress in the development of new methods for the synthesis of diamond crystals and in their post-growth treatment, many questions remain unclear about the conditions for the formation and degradation of aggregate impurity nitrogen forms. Meanwhile, they are very important for understanding (evaluating) the origin, age, and post-growth conditions of natural diamonds. In the present work, an attempt was made to analyze the causes of the formation of high concentrations of N3V centers in natural IaB-type diamonds from the Kholomolokh placer (the Northeast Siberian craton). The possibility of decay of B centers during the plastic deformation of diamonds is analyzed and experiments on the high-temperature annealing of diamonds containing B centers are reported. The formation of N3V centers during the destruction of the B centers at high-pressure annealing of crystals has been established by experiment. It is assumed that, in the post-growth period, diamond crystals were exposed to tectono-thermal stages of raising the superplumes of the Earth's crust of the Siberian craton.

It is widely accepted that granulite xenoliths from kimberlites provide a record of granulite facies metamorphism at the basement of cratons worldwide. However, application of the phase equilibria modeling for seven representative samples of mafic granulites from xenoliths of the Udachnaya kimberlite pipe, Yakutia, revealed that a granulitic garnet + clinopyroxene + plagioclase ± orthopyroxene ± amphibole ± scapolite mineral assemblage was likely formed in the middle crust under amphibolite facies conditions (600–650 °C and 0.8–1.0 GPa) in a deficiency of fluid. Clinopyroxene in the rocks is characterized by elevated aegirine content (up to 10 mol.%) both in the earlier magmatic cores and in the later metamorphic rim zones of the grains. Nevertheless, the phase equilibrium modeling for all samples indicates surprisingly reduced conditions, i.e. oxygen fugacity 1.6–3.3 log units below the FMQ (Fayalite-Magnetite-Quartz) buffer. In contrast, the coexistence of Fe-Ti oxides indicates temperatures of 850–990 °C and oxygen fugacity about lg(FMQ) ± 0.5, conditions which correspond to earlier stages of rock evolution. Reduction of oxygen fugacity during cooling is discussed in the context of the evolution of a complex fluid. The reconstructed P-T conditions for the final equilibration in the mafic granulites indicate that temperatures were ~250 °C higher than those extrapolated from the continental conductive geotherm of 35–40 µW/m2 deduced from peridotite xenoliths of the Udachnaya pipe. Although the granulites resided in the crust for a period for at least 1.4 Ga, they did not re-equilibrate to the temperatures of the geotherm, likely due to the blocking of mineral reactions under relatively low temperatures and fluid-deficient conditions

Polycrystalline diamond aggregates (boart, framesites, diamondites) have been widely studied but their origin is poorly understood. We report the results of a study in situ of two polished fragments of fine-grained (40–400 μm size of individual diamond grains) dense polycrystalline diamond aggregates from the Mir pipe containing visible multiple interstitial garnet inclusions. They were analyzed for major and trace elements of inclusions and one of them — for δ 13 C and N abundance and isotopic composition of host diamonds. These aggregates are classified as variety IX by Orlov (1977). No cavities were observed in these samples. Sixty two irregular garnet grains and one clinopyroxene inclusion were detected and analyzed in sample Mr 832. Garnets are homogeneous within single grains but variable in Mg# [100Mg/(Mg + Fe)] from 60 up to 87 and CaO contents (3.3–5.3 wt.%) among grains with a trend to negative correlation. Low Cr (550–640 ppm) confirms eclogitic (E-type) paragenesis. High Na 2 O contents (5.2 wt.%) of a single pyroxene inclusion are additional evidence of eclogitic nature of this sample. Wide variations in trace elements (ppm) are characteristic for garnet grains: Sr (2.7–25.6), Y (9.7–14.1), Zr (15.6–38.7) and positive Eu anomaly is present. The δ 13 C of diamonds within studied sample is variable (− 6.4 ÷− 9.8 ‰) as well as N abundance (75–1150 ppm) and δ 15 N − 27, − 38, − 58 ‰. The second peridotitic (U/P-type) sample Mr 838 contains eight inclusions of Mg-rich Cr-pyropes (Mg# ~ 85, Cr 2 O 3 3.2–3.4 wt.%) and magnesite inclusion with 4.35 wt.% FeO and 1.73 wt.% CaO. Trace element content in pyropes is relatively uniform (ppm): Sr (0.4–1.6), Y (13.2–13.4) and Zr (13.0). We conclude that heterogeneous distribution of the trace elements among garnet grains in Mr 832 and magnesite presence in Mr 838 are indicative of the effects of mantle metasomatism and rapid crystallization shortly before the eruption of the kimberlite.

U–Pb age and Hf-isotope data were collected on zircons from crustal xenoliths from the Late Devonian kimberlite pipes of Markha terrane. The xenoliths include mafic garnet granulites (Gt+Pl+Cpx±Opx±Amp±Bt±Scp) and garnet–biotite gneisses (Grt+Bt+Pl+Kfs+Qtz±Scp). The data from this study demonstrate that the crust of the Markha terrane experienced several tectonothermal events. Zircons with Archean Hf model ages (TDM = 3.13–2.5 Ga) are predominant in all xenoliths with one exception. These data imply that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time. Later this Paleoarchean crust was significantly reworked in several tectonothermal events, including a Neoarchean stage (2.9–2.5 Ga), and several Paleoproterozoic metamorphic stages (1.98, 1.9 and 1.8 Ga). There is very little evidence for generation of juvenile crust associated with these metamorphic events. Importantly, crustal xenoliths from kimberlite pipes of the Markha terrane record all the same events that occurred in the adjacent Daldyn and Magan terranes.

The specific gray to almost black diamonds of rounded morphology are especially typical in alluvial placers of the northeastern part of the Siberian platform. The results of study of internal structure of these diamonds are presented. X-ray topography and birefringence patterns of polished plates of studied diamonds show their radial mosaic structure. Diamonds consists of slightly misorientated (up to 20′) subindividuals which are combined to mosaic wedge-shaped sectors. Electron back-scatter diffraction technique has demonstrated that subindividuals are often combined in the single large blocks (subgrains). The whole crystals commonly consist of several large subgrains misoriented up to 5° to one another. The total nitrogen content of these diamonds vary in the range 900–3300 ppm and nitrogen aggregation state (NB/(NB + NA)*100) from 25 to 64 %. Rounded diamond crystals of variety V are suggested to have been formed at the high growth rate caused by the high oversaturation of carbon in the crystallization medium. It may result in the splitting of growing crystal and their radial mosaic structure as a sequence. High content of structural nitrogen defects and the great number of mechanical impurities – various mineral and fluid inclusions may also favor to generation of this structure.

The Baikal–Muya Foldbelt is one of the oldest and the most enigmatic terrain among the Central Asian Orogenic Belt. During the Early (1.0–0.8 Ga) and Late (0.8–0.6 Ga) Neoproterozoic orogenic cycles a notable amount of juvenile crust has been formed in subduction–collision settings along with the reworking of the existing Early Precambrian continental crust. The latter is perfectly presented within the North Muya metamorphic block (Anamakit–Muya zone) including the oldest high-pressure subduction-related rocks in the Central Asia. In order to evaluate the process of the juvenile crust formation in the Precambrian continental unit, whole-rock trace elements along with zircon U–Pb ages and Hf-isotopic composition were analyzed in granites of unknown age that intrude the Precambrian volcanic-sedimentary rocks of the Parama series in the southern North Muya Block. Combined geochemical and zircons isotopic studies indicate the formation of granites with the main pulse at ∼810 Ma due to partial melting of a juvenile mafic crust accompanied by reworking of the older crustal material from the continental arc basement. The composition of the granites was controlled by processes of intracrustal differentiation together with a probable contamination by older crustal substrate. Geochemistry and age of the studied granites are comparable to the granites of the Muya complex observed in the volcano–plutonic Kelyana–Irokinda zone. It is assumed that the Muya block and the Kelyana–Irokinda zone (terrane) had possibly been parts of the large continental-arc system beyond the Siberia margins.

Diamonds of eclogitic assemblages are dominant in the placer diamond deposits of the northeastern Siberian platform. In this study we present new trace elements and stable isotopes (δ13C and δ18O) data for alluvial diamonds and their garnet inclusions from this locality. Cr-rich garnets of peridotitic affinity in the studied diamonds have a narrow range of δ18O values from 5.7‰ to 6.2‰, which is largely overlapping with the accepted mantle range. This narrow range suggests that the garnet inclusions showing different REE patterns and little variations in oxygen isotopes may have formed by different processes involving fluid/melts that, however, were in oxygen isotopic equilibrium with the mantle. The trace element composition of the eclogitic garnet inclusions supports a crustal origin for at least the high-Ca garnets, which show flat HREE patterns and in some cases a positive Eu-anomaly. High-Ca eclogitic garnets generally show heavier oxygen isotope compositions (δ18O 6.5–9.6‰) than what is observed in low-Ca garnets (δ18O 5.7–7.4‰). The variability in oxygen isotopes and trace elements is suggested to be inherited from contrasting crustal protoliths. The relationship between the high δ18O values of inclusions and the low δ13C values of the host diamonds implies that the high-Ca garnet inclusions were derived from intensely hydrated (e.g., δ18O > 7‰) and typically oxidised basaltic rock close to the seawater interface, and that the carbon for diamonds was closely associated with this protolith.

We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfs, sulfide) and peridotitc (olivine) paragenesis. Iron carbides with minor amounts of admixed nickel were detected in a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by a relatively low nitrogen aggregation state (5–35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere. Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).

Photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) were used for the first time to characterize properties of superdeep diamonds from the Sao-Luis alluvial deposits (Brazil). The infrared measurements showed the low nitrogen content (>50 of 87 diamonds from this locality were nitrogen free and belonged to type IIa) and simultaneously the extremely high level of nitrogen aggregation (pure type IaB being predominant), which indicates that diamonds under study might have formed under high pressure and temperature conditions. In most cases, PL features excited at various wavelengths (313, 473, and 532 nm) were indicative of different growth and post-growth processes during which PL centers could be formed via interaction between vacancies and nitrogen atoms. The overall presence of the 490.7 nm, H3, and H4 centers in the luminescence spectra attests to strong plastic deformations in these diamonds. The neutral vacancy known as the GR1 center has probably occurred in a number of crystals due to radiation damage in the post-growth period. The 558.5 nm PL center is found to be one of the most common defects in type IIa samples which is accompanied by the EPR center with g-factor of 2.00285. The 536 and 576 nm vibronic systems totally dominated the PL spectra of superdeep diamonds, while none of “normal” diamonds from the Mir pipe (Yakutia) with similar nitrogen characteristics showed the latter three PL centers.

U–Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite–gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet–biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet–biotite–muscovite schist (Pl + Kfsp + Bt + Mu + Grt + Qtz) were dated using the LA-ICP-MS technique. Based on U–Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576–680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U–Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88–2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a 207 Pb/ 206 Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of ɛ Hf ( t ) values (from +13.9 to −15.3) and T DM C model ages (0.82–3.86 Ga). Zircons from other samples have a much narrower ranges of ɛ Hf ( t ) (+11.6 to −0.7) and T DM C (0.85–1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with ɛ Nd ( t ) values ranging from −3.5 to +3.6 and t Nd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.

Diamonds from placer deposits in the northeastern Siberian platform were examined for variations in the isotopic composition of carbon and concentrations of nitrogen. The diamonds display large variability in N concentrations, ranging from below detection to 3500 ppm. Nitrogen levels in diamonds with eclogitic inclusions are generally high (average of 950 ppm) compared to diamonds of the ultramafic suite (average of 513 ppm). Diamonds belonging to variety V (classification scheme of Orlov, 1977) have relatively high N levels (from 1500 to 3500 ppm, average of 2549 ppm). The average nitrogen level is five times higher than the worldwide average. Values of δ 13 C in the diamonds range from −27.2 to −3‰ ( n = 28) in eclogitic diamonds and from −7.1 to −0.5‰ ( n = 16) in peridotitic diamonds. Diamonds of variety V range in δ 13 C from −24.1 to −17.4‰. The distributions of nitrogen and its aggregation state in some diamond crystals imply the occurrence of multiple growth events. This is especially evident in the nitrogen content, which decreases and then increases from core to rim. The results indicate that there is no correlation between local variations in the isotopic composition of carbon and either the content or degree of aggregation of nitrogen. The diamonds of variety V differ from most diamonds worldwide in their high N content and light C-isotope composition. The large ranges of δ 13 C and the lack of correlation with nitrogen levels are inconsistent with the open-system Rayleigh isotopic fractionation of carbon species. The nature of the variations in the carbon isotope composition and the nitrogen concentrations indicate that the diamond growth medium had at least two sources of fluids/melts (mantle and recycled Earth crust via subduction). Mantle carbon was involved in the process of diamond formation during the final stages of diamond growth.

Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca 2.85 Mg 0.96 Fe 0.11 Si 2.04 O 8 ) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the Sao Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO 3 in the core and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO 3 and merwinite) and diamond at an early stage, and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

To clarify the tectonic-thermal evolution of the Anabar tectonic province in the central Siberian Craton, we performed an isotope-geochemical study of 20 xenoliths from the Udachnaya, Zarnitsa, and Komsomolskaya kimberlite pipes to represent different crustal levels. Most mafic granulites have Proterozoic Nd model ages and geochemical characteristics close to those of intraplate basalts, whereas some mafic and intermediate granulites with Archean model ages exhibit geochemical features of supra-subduction ophiolitic basalts. Analysis of U-Pb ages and hafnium isotopic composition of zircon indicates that the main tectonic-thermal events modified the crust at 2.7 and 1.9–1.8 Ga, which is consistent with ages of mantle depletion events from previous studies. All zircons have Archean Hf model ages ( T DM C 3.6 - 3.2 Ga). Overall, thermal events with ages of 2.9–2.8, 2.7, 2.4, 1.97 and 1.8 Ga have remarkable influence on the studied zircons. Tectono-thermal events at 2.4 1.97, 1.9 and 1.8 Ga with no addition of juvenile material are recorded by zircons from xenoliths of mafic and intermediate granulites and metadiorites. A compilation of isotope-geochemical data demonstrates that instead of age-stratified, the crust of the Anabar tectonic province consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence the crust and mantle of the Siberian Craton has been coupled since the Paleoarchean.

Taking advantage of the common occurrence of superdeep mineral inclusion assemblages, we examined core-to-rim primary zonation in 69 diamonds from the alluvial deposits of the Sao-Luis River (Juina, Brazil). Syngenetic inclusions were represented by phases of superdeep paragenesis. The dominant inclusions are majoritic garnets, ferropericlases and CaSi- and CaSiTi-perovskites. Rare inclusions of MgSi-perovskites, olivines, clinopyroxenes, TAPP, phases of SiO2, kyanites, AlSi-phases, KFsp (K-hollandite?), CF, NAL, grossular, merwinite, native iron, Fe-sulphides, magnesite and CaCO3 + CaMgSi2O6 (composite inclusions) were also found. The diamonds from Sao-Luis display wide variations in carbon isotopic compositions (δ13C), from +2.7 to −25.3‰. The diamonds with inclusions of ferropericlase have a very narrow range of δ13C values, from −2.1 to −7.7‰, which are close to the “normal” mantle values. In many cases, diamonds with inclusions of calcic majoritic garnet and CaSi- and CaSiTi-perovskites display marked differences from this range. Low δ13C values (−10 to −25 ‰) were observed exclusively in a series of diamonds with majorite garnets, Ca-silicates, aluminous silicates and SiO2. The δ13C values from the cores to rims in certain individual crystals varied substantially, indicating multiple stages of growth. The highly negative δ13C values in the cores (−20 to −25 ‰) potentially represent organic matter in metasediments of altered oceanic crust, and the lower δ13C values may represent mixing trends towards “normal” mantle compositions. In this study, we also found a set of diamonds that display an opposite trend of change of the carbon source, from primordial mantle to subducted/crust (either biotic or abiotic carbon).

Comprehensive studies of diamond crystals with a low nitrogen concentration from the placer deposits and kimberlite pipes of Yakutia have facilitated the detection of three titanium-nitrogen-related centers: OK1/S1, N3/440.3 nm and NU1/485 nm in the EPR and photoluminescence spectra. Additional proof of titanium presence in the structure of EPR N3 center was obtained from the hyperfine structure of titanium magnetic isotopes 47 Ti and 49 Ti. Titanium-nitrogen-related centers in the neighboring substitutional sites (N3/440.3 nm) and in the structure of the interstitial (NU1/485 nm) are not found in cubic habit crystals, which are characterized by the presence of the OK1/S1 center with a double semi-vacancy structure. All three types of nitrogen-titanium-related centers are present in cuboctahedral habit diamonds. It is assumed that these habit-specific distributions of defects in diamond are a result of the peculiarities of incorporating nitrogen and titanium impurities into the different growth sector of crystals and aggregation thereof at high pressure and temperature.

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine-grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse-grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four-stage metamorphic evolution: (I) pre-eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre-eclogite stage or coronal eclogite, and so did not identify the four-stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE-enriched patterns [(La/Lu)N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO2 (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO2

article i nfo The world's largest diamondiferous eclogite (8.8 kg, UD-111/02) from the Udachnaya kimberlite, Siberia, is the subject of this investigation. High-resolution X-ray computed tomography (HRXCT), chemistry of host minerals and diamond inclusions, stratigraphy of the diamonds (cathodoluminescence zoning), and carbon isotopes have been obtained. HRXCT images, 3-d models, and subsequent detailed dissection of this Group B eclogite revealed linear distributions of diamonds along alteration zones. Both unusual multiple-diamond clusters with dodecahedral morphologies, and distinctive internal zoning recorded by cathodoluminescence analyses are indicative of multiple stages of diamond growth and resorption in UD-111-02. Carbon-isotope ratios (δ 13 C) of 16 diamonds have a total range of ~7‰ (−2.0 to −9.2‰), with the majority at −5.4‰.

Article history:Received 23 January 2008Accepted 29 June 2008Available online 10 July 2008Keywords:SHRIMP dataU/PbZirconKokchetavUHP metamorphismpartial melting The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality amongnumerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocksare interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granitegneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion,sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at theKumdy-Kol regionwas performed to constrain the age of partial melting of the Kokchetav UHP metamorphicrocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206 Pb/ 238 Uages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses ofzircons from four samples fall in the range 508–538 Mawith the weighted mean age for all zircon domains at526±2.1 (MSWD=1.7). Our data show that migmatization of UHP pelites occurred later than the peakmetamorphism (537±9 Ma) and the decompression partial melting took place during exhumation ofdiamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507±8 Ma).© 2008 Elsevier B.V. All rights reserved.

We present the results of a detailed investigation of large (18 cm long, 6.9 kg) diamondiferous eclogite xenolith from the Udachnaya kimberlite pipe. We have studied the spatial distribution of diamonds, their relationship with the matrix minerals, mineral inclusions and their major- and trace-element compositions. The xenolith is composed of pale green clinopyroxene, round garnet grains and phlogopite, and is characterized by irregular distribution of minerals and development of zones enriched in phlogopite and garnet. Minerals in diamondiferous xenoliths show major-element compositions transitional between Udachnaya eclogites and those from peridotites. The host clinopyroxene are enriched relative to pyroxenes from Group A eclogites and websterites in respect K 2 O and LREE. We believe that such peculiarities reflect the interaction between mantle peridotites and LREE-enriched melts. The sources of the melts are uncertain. The distribution of diamonds (1 to 5 mm in size) in the xenolith is extremely irregular: they are more abundant in the garnet- and phlogopite-enriched zones. Diamonds often make up veins and schlieren. Nitrogen aggregation characteristics suggest that the xenolith contains diamonds of at least two generations. High degrees of aggregation in colorless diamonds of the earlier generation indicates that they formed at least 1 Ga before kimberlite eruption, whereas yellow diamonds of the second generation (type IaA) were formed only a short time before kimberlite emplacement. The presence of K-Clinopyroxene, coesite, CaCO 3 , and high-silica mica inclusions in diamonds of the second generation, and their internal and external morphology, suggest that these diamonds crystallized during interaction of the eclogitic substrate with high-density fluids compositionally intermediate between carbonatitic and hydrous-silicic end members rich in Si, K, Ti.

The bulk major- and trace-element compositions of micro-inclusions in 24 cuboid diamonds from the Udachnaya kimberlite pipe (Siberia, Russia) have been quantitatively analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LAM-ICPMS). Micro-inclusions in the studied diamonds represent a bulk sample of the fluids from which the diamonds crystallized; they define a continuous range between magnesian and calcic carbonate-rich compositions at relatively constant Fe contents (Ca/(Ca + Mg) = 0.31–0.68; Mg/(Mg + Fe) = 0.66–0.78). In general the major- and trace-element patterns of the trapped fluids in Udachnaya cuboid diamonds are similar to those of kimberlites and carbonatites. However, some important differences are observed: (1) the fluids in diamonds are enriched in K, Na, Fe and depleted in Al relative to the host kimberlite; (2) the REE patterns of the fluids are steeper than those of kimberlites; (3) many of the fluids show strong depletion in Ti, Zr and Y. The observed geochemical features are consistent with a genetic link between the diamond-forming fluids and ephemeral carbonatitic fluids/melts which may be precursors of the host kimberlite. These fluids/melts may originate either from the metasomatic and influx of carbonatitic agents or from partial melting of previously carbonated eclogites and peridotites. Some elemental variations may be explained by fractional crystallization of such fluid/melts, or mixing between fluids with different compositions.

In this communication we present an estimation of the concentration of single substitutional nitrogen atoms (P1 centre) from EPR data in Kokchetav diamonds. We first performed an analysis of local P1 centre concentrations using van Wyk’s treatment (van Wyk et al ., 1997) of the line width of the resolved EPR spectrum. Also by analysing the dependence of the line-width of the broad exchange line on the P1 concentration in range higher than 1000 ppm in synthetic diamonds, we obtained a calibration curve allowing the estimation of the local P1 centre concentration higher than 1000 ppm and the local volumes occupied by substitutional nitrogen atoms without taking measurements of microcrystalline volume. In addition to the development of a method for calculating high local nitrogen concentrations, the aggregation process in P,T-treated Kokchetav diamonds has been given considerable attention and it was shown that the rate of nitrogen aggregation in these diamonds is consistent with data given in Evans (Evans & Qi, 1982).

Water-related features were studied using infrared absorption spectroscopy in fibrous diamonds with micro-inclusions. Both OH-stretching and HOH-bending vibrations were observed. The lack of correlation between the intensities of HOH and OH bands in different samples and the complexity of the OH-stretching band indicate that a large fraction of water is present as hydroxyl groups in minerals.Heating and cooling experiments were performed to elucidate the properties of fluids in micro-inclusions in diamonds. The results of experiments at various temperatures support the presence of several water-related components in individual diamonds. These spectroscopic investigations of fibrous diamonds revealed two or more interrelated water-related components preserved in micro-inclusions. The existence of several water phases or water solutions with different salinity and solutes within one diamond crystal is possible.

A progression from cuboid to octahedral growth has been observed in 16 natural diamonds from Yakutian kimberlites. X-ray and cathodoluminescence topography have revealed that the change in morphology of diamonds with cloudy cuboid cores may occur without mixed-habit growth but via generation of numerous octahedral apices on cuboid surfaces and subsequent gradual transformation into regular octahedral morphology. Nitrogen aggregation in both cuboid and octahedral domains of such diamonds suggests that they have had a long residence time under mantle conditions. Micro-inclusions in the cuboid domains of the diamonds testify to the nucleation and growth of cuboid cores from a hydrous-carbonatitic (oxidized) fluid. The transition from cuboid hummocky growth rich in inclusions to octahedral growth without inclusions may be linked to decreasing supersaturation in the parent fluid. Measurements of δ13C and Nppm by ion microprobe show that the chemical variations observed between inner cuboid domains and outer octahedral zones commonly have a systematic character and as such they are probably not due to purely kinetic effects. The peripheral octahedral zones are always enriched in 13C in comparison with inner cuboid ones, and the total nitrogen content decreases with the change from cuboid to octahedral growth. The octahedral outer zones show a gradual progressive increase in δ13C, with an overall change of up to 5‰ from the cuboid core (δ13C usually between −8 and −6‰) to the diamond margin (δ13C usually between −4 and −2‰). Decreases in δ13C of this magnitude with a gradual increase in 13C may be attributed to the Rayleigh fractionation operating on a single parent fluid of close to normal mantle δ13C composition with diamond precipitating by the reduction of carbonatitic fluid in a closed system. However, one sample shows a variation of δ13C of approximately −17 to −6‰ and therefore suggests a possible change of fluid source composition from one containing subducted crustal organic carbon to one with common mantle carbon.

A highly potassic COH fluid, with dissolved phosphate, chloride and sulfate/sulfide, was identified as nanometer-size inclusions in metamorphic microdiamonds of Kokchetav dolomite marble and garnet–quartz–pyroxene rock using an analytical electron microscope (AEM) coupled with an energy-dispersive X-ray (EDX) spectrometer. This provides a direct piece of evidence for the formation of metamorphic microdiamonds from such a fluid. Although this crust-derived potassic fluid and the associated solid precipitates are in some respects similar to the fluid inclusions in fibrous/cloudy mantle diamonds, distinct chemical differences exist. This potassic COH fluid, in addition, has the potential to trigger high to ultrahigh-K magmatic and K metasomatic processes in mantle.

The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100–120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds. Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°–80°) thin slices (few hundred metres) within granite–gneiss. Using geological, petrological and isotopic–geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models. Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle. We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift.

Kokchetavite, a new polymorph of K-feldspar (KAlSi3O8), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) A, c=7.82(1) A, V=188.09 A3, Z=1, and is found to be associated with phengite + α/β-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar.

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Micro-inclusions (1 –10 μm) in 55 diamonds of cubic habit from the Udachnaya kimberlite pipe have been studied using vibrational spectroscopy. This has revealed a multiphase assemblage in cuboid diamonds from the Udachnaya kimberlite pipe. This assemblage includes carbonates, olivine, apatite, graphite, water and silicate glasses. The micro-inclusions preserve the high internal pressure and give confidence that the original materials were trapped during growth of the host diamond. The internal pressures, extrapolated to mantle temperatures, lie within the stability field of diamond and the relatively low temperatures are typical for the formation of cuboid diamonds. In contrast to previously reported data for African diamonds, the micro-inclusions in the cuboids from Udachnaya are extremely carbonatitic in composition (H2O/(H2O+CO2) ≈5 –20%) with the observed assemblage of microinclusions similar to some types of carbonatites. The low water and silica content testify that the material in the micro-inclusions of the Udachnaya diamonds was near-solidus carbonatitic melt. Vibrational spectroscopy has provided the evidence of carbonatitic melts in cuboid diamonds.

Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopies have been used to study Brazilian and Central African carbonados. We report the first observation of the N 2 V + and N + 2 nitrogen defect centers in carbonados. The fact that Brazilian carbonados contain the N 3 V (previously reported), N 2 V + , and N + 2 defects, the formation of which requires complicated aggregation of nitrogen, indicates histories at elevated temperature and pressure. The observation of both the radiation-induced Al defect, which has an annealing temperature of 700 K, and radiation defects containing two and/or five vacancies, suggest a continuous process of the formation and transformation of radiation defects in the carbonado under variable pressure and temperature conditions while in the upper mantle. The temperature at which these defects are formed varies from 300 to 1500 K. In addition we report a hydrogen-containing defect labeled H1, previously only observed in synthetic diamonds prepared by chemical vapor deposition. Together with the previously reported light carbon isotope ratio, the results of this study confirm the hypothesis that carbonados are derived from hydrocarbon source material.

Clinozoisite gneisses were studied from the Barchi-Kol area, located 17 km to the west of the Kumdy-Kol microdiamond deposit at Kokchetav massif (Northern Kazakhstan). As distinct from the deposit, the studied rocks are characterized by predominance of diamonds of octahedral habit. Ultrahigh-pressure mineral assemblages were investigated mainly as inclusions in zircons from these rocks. It is established that nucleation and growth of some zircon grains began at the peak of metamorphism (T = 950–1000°C and P > 40 kbar) and continued while temperature and pressure decreased to T = 650–750°C and P = 10–12 kbar, Ultrahigh-pressure metamorphic conditions for clinozoisite gneisses from the Barchi-Kol area are comparable with those for the Kumdy-Kol microdiamond deposit and correspond to T = 900–1000°C and P > 40 kbar. High P-T of metamorphism and bulkrock composition did not affect the morphology of the diamond crystals. The abundance of fluid or melt is proposed to be responsible for the variable extent of the completeness of cuboid reshape reaction, resulting in the formation of octahedral diamond crystals. The preservation of coesite as inclusions within zircon grains, as well as within garnets, suggests the rapid cooling and fast exhumation of the studied rocks. Based on concentration profiles in garnets from diamondiferous clinozoisite rocks, the duration of retrograde metamorphism is estimated to be less than 0.1 Ma.

This study reports δ13C, δ15N and N-content values for microdiamonds from ultrahigh-pressure metamorphic rocks of the Kokchetav massif in Kazakhstan. Both alluvial diamonds and in-situ diamonds from a garnet–clinopyroxene rock and a marble (i.e. a garnet–pyroxene dolomitic rock) were investigated. In-situ diamonds were analysed in batches, because of their small size (average 40 μm), whereas the larger alluvial diamonds were analysed individually. The latter group has δ13C-values ranging from −15.92‰ to −10.57‰, δ15N from −1.8‰ to +1.1‰ and N-contents from 2300 to 3650 ppm. Diamonds from the garnet–clinopyroxene rock yield mean values of −10.50‰ for δ13C, +5.9‰ for δ15N and a high average nitrogen content of 11,150 ppm. Values for diamonds in marble are −10.19‰, +8.5‰ and 2650 ppm, respectively. For diamonds from garnet–clinopyroxene rock and marble, there is more nitrogen released by bulk combustion than estimated by infrared (IR) spectroscopy, the differences being of about 7000 and 1500 ppm, respectively. These differences suggest that a significant quantity of nitrogen is IR-inactive and may be present as fluid inclusions. Their carbon and nitrogen isotopic compositions are compatible with an in-situ crystallisation of diamond from dominantly metasedimentary sources, suggesting that sedimentary nitrogen can be subducted to very high pressures. Carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures before the peak of metamorphism at temperatures probably below 700°C and deduced pressures of 3 GPa. Relative to the isotopic data reported for sediments, metasediments and in-situ diamonds, the slightly 15N-depleted compositions of alluvial diamonds is striking. These values suggest that the contribution of any metasedimentary source is unlikely and may point toward a mafic/ultramafic protolith.

Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events.

Microdiamonds from garnet clinopyroxenites of the Kokchetav massif (northern Kazakhstan) and associated alluvial diamonds have been investigated using infrared spectroscopy. All diamonds have high nitrogen contents (from 747 up to 4488 ± 20% at. ppm) and are of type Ib-IaA. Nitrogen data are consistent with a diamond formation over a narrow temperature range. Microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions, similar to those ocurring in fibrous coated diamonds from kimberlite, which suggest that diamonds grew from a C-H-O fluid. In contrast, alluvial microdiamonds do not contain any carbonate inclusions and H2O is absent or present in only minor amounts. Alluvial diamonds are considerably more N-rich but have a similar range of nitrogen aggregation states. It is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites. The Kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant Ib centres, have high nitrogen contents and show different populations depending on host rock type. These characteristics support a metamorphic origin for the microdiamonds from the UHPM rocks of the Kokchetav massif.

Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P–T path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na2O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K2O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800–1000 °C at P >40 kbar for diamond-bearing rocks, and about 740–780 °C at >28–35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650–680 °C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420 °C and 2–3 kbar. A clockwise P–T path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.

Garnet crystals from diamondiferous high-pressure metamorphic rocks of the Kokchetav Massiv, Kazakhstan, were studied by electron microprobe and single crystal microscope-spectrometry in the UV/VIS and IR spectral ranges. The crystals are quaternary solid solutions Gross 48 Pyr 33 Alm 16 Spess 2 , (I.) from garnet-pyroxene-carbonate rock K7, and Gross 21 Pyr 24 Alm 49 Spess 5 , (II.) from garnet-biotite-gneiss, both with small TiO 2 -contents (0.3 wt%). Their yellow (I.) or deep-red colour (II.) is caused by the position of the UV absorption edge and a broad Fe 2+[8] Ti 4+[6] charge-transfer band centered at 21 500 cm −1 . For comparison, a high-pressure garnet Py26, Py 70 Alm 17 Uv 13 (III.), from Liaoning-50 kimberlite, NE China, was included in the IR and microprobe study. All crystals studied contained defect OH − -groups, corresponding to those in synthetic OH − -bearing pyropes, giving rise to a single, relatively sharp absorption band (Δv 1/2 ≃ 100 cm −1 ) in the range 3560 to 3610 cm −1 . The exact energy position depends on the type and site fractions of ions in the OH − -coordinating metal sites, A [8] and B [6] . This hydroxyl «water» seems to be not uniformly distributed in the garnet crystals and ranges from 0.02 to 0.25 wt% (wt% calculated from integral intensities using e i,Gross ). In addition to ν OH , the IR spectra show a broad band at 3400 cm −1 , characteristic of (H 2 O) n -clusters. There seems to be a constant ratio α i,OH− /(α i,OH− + α i,H2O ) for the various spots on the different crystals I, II, and III

The behaviour of rare-earth elements (REE) during high-pressure metamorphism has been studied to estimate their possible use in determining the geodynamic settings of eclogite procoliths. Eclogites exhibiting various degrees of amphibolitization have been studied in a number lf metamorphic complexes in the Urals-Mongolian fold belt (the Kokchetav massif in northern Kazakhstan and the Atbashi range in the Central Tien-Shan, all in the U.S.S.R.). In addition, an eclogitized metagabbro from the Koralpe, Austrian Alps and gabbroids transformed into garnet amphibolites from the Kokhchetav massif have been investigated. The analytical data suggest that about 30–60% of the light rare-earth elements (LREE) in eclogites is present in the intergranular space. The results of the investigation demonstrate that the REE were virtually immobile during eclogite formation. Their low mobility can be explained either by absence of complex formers in the fluid phase or by low fluid/rock ratio.

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T~950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P~10 kbar; T~800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P~5 kbar; T~600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527±5, 528±8 and 526±5 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528±3 Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P–T–t path. Fast exhumation on top of the subducted slab from depth >140 to ~35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.